1. Technical Field
The field addressed is that of optical fiber communication entailing optical amplification. It specifically concerns optical pumping of an erbium fiber amplifier for single channel or multiple channel (wavelength division multiplex) use.
2. Description of the Prior Art
Most advanced optical fiber communication systems now in place and contemplated owe success and operating characteristics to the erbium-doped fiber amplifier ("EDFA"). Due to its low noise figure, it becomes practical to increase span lengths between regenerators to suit most terrestrial and underwater needs. The gain bandwidth of this excellent amplifier is sufficient to permit simultaneous amplification of multiple channels and, for this reason lends impetus to wavelength division multiplexing (WDM).
EDFA's are commonly pumped at about 1480 nm--the emission wavelength of a commercially available laser. While acceptable, there are advantages to be realized by pumping the amplifier with a source matched to the 980 nm erbium absorption line. For one thing, amplifier noise is inherently lowered. A further advantage is realized for WDM systems of the future. The gain spectrum of the amplifier over the needed wavelength region to encompass the separate channels is substantially wavelength-independent.
The InGaAs strained quantum well laser has been identified as a suitable 980 nm pump source for the erbium amplifier. In addition to satisfying contemplated advantages for amplifier operation, its improved electrical-to-optical conversion efficiency at this wavelength results in a power saving and reduced heating. See, S. Uehara, "Pump sources for erbium doped fiber amplifiers", Technical Digest on Optical Amplifiers and Their Applications, 1990 (Optical Soc. of America) vol. 13, Paper WA1, pp. 206-209.
Operation at this pump wavelength, however, raises a new problem. Pump wavelength stabilization at 1480 nm has been assured by use of an optical isolater. Corresponding 980 nm devices are relatively lossy.
The problem is aggravated by a growing desire to provide for two or more tandem lasers for increased power and redundancy. Without an isolator laser interactions may cause fluctuations in power level.